1. Field of the Invention
The present invention relates to a disc brake actuating apparatus having a parking-brake operating mechanism provided at one end portion of a caliper slidably supporting a piston containing an adjuster mechanism.
2. Related Art
As conventional parking-brake operating mechanisms of a disc brake actuating apparatus, there are known the type employing toggle links, the type employing a ball ramp and the type employing a cam. In any of these types, however, it is difficult to obtain a high boost ratio within the range of an allowable brake lever stroke. In order to obtain high boost ratio with a limited brake lever stroke, JP-A-60-065920 discloses a parking-brake operating mechanism in which toggle links are arranged symmetrically with respect to a point. Further, a type of parking-brake operating mechanism in which an inclination of a ramp surface in a ball ramp is made non-linear is also known.
FIG. 13 is a cross-sectional view of a portion of a parking-brake operating mechanism disclosed in JP-A-60-065920. When a parking lever 1 is driven to be pivotally moved in a direction of an arrow, a cam shaft 2, provided at one end of the parking lever 1, is angularly moved, and one toggle link 3 (which is one of the two toggle links arranged symmetrically with respect to a point) moves or pushes out an adjusting spindle 4 in a left-hand direction to operate a brake piston (not shown), and at the same time the other toggle link 5 is moved or pushed out in a right-hand direction to operate a caliper 6, so that a pair of brake pads are pressed respectively against opposite sides of a brake disc with a high boost ratio, thereby effecting a braking operation. By sliding holes 6a and 6a which are formed within the caliper 6, and have width across flats, the cam shaft 2 is supported for movement in a direction parallel to an axis of the adjusting spindle 4, and therefore a rotational frictional resistance of the cam shaft 2 can be reduced, and therefore this construction has been effective.
However, in the parking-brake operating mechanism employing such toggle links, sliding holes 6b and 6b for the sliding movement of the adjusting spindle 4, as well as the sliding holes 6a and 6a (which have the width across flats spaced from each other in a direction perpendicular to the adjusting spindle 4) for the sliding movement of the cam shaft 2, must be formed within the caliper 6. The formation of the sliding holes 6a (having the width across flats for the cam shaft 2) by grinding from the outside of the caliper 6 (This process is separate from a process of forming the sliding holes 6b) has been relatively difficult from the viewpoint of a machining technique, and has required much time. And besides, although the toggle link 3 is slidably supported between the cam shaft 2 and the adjusting spindle 4 while the toggle link 5 is slidably supported between the cam shaft 2 and the caliper 6, a frictional resistance of a contact portion of each toggle link increases when an axial thrust increases, and this has led to a power transmission loss.